The present invention relates to an exhaust emission control system of an engine, and particularly to an exhaust emission control system which is provided on an exhaust passage with a NOx catalyst which purifies NOx in exhaust gas.
Conventionally, NOx storage catalysts which store (occlude) NOx contained in exhaust gas when an air-fuel ratio of the exhaust gas is lean (i.e., λ>1, larger than a theoretical air-fuel ratio) are known. Such NOx storage-reduction catalysts further reduce the stored NOx when the air-fuel ratio is approximately equal to stoichiometric (i.e., λ≈1, approximately equal to the theoretical air-fuel ratio) or is rich (i.e., λ<1, smaller than the theoretical air-fuel ratio). Within a normal operating range of an engine, the engine is operated at the lean air-fuel ratio (λ>1) so as to reduce fuel consumption, although if this lean operation state continues for a while, the NOx stored amount in the NOx catalyst reaches a limit value and the NOx catalyst can no longer store NOx, which causes NOx to be released. For this reason, the air-fuel ratio is suitably set to be stoichiometric or richer (λ≤1) in order to reduce NOx stored in the NOx catalyst (hereinafter, the control for reducing NOx stored in the NOx catalyst is referred to as “NOx reduction control”). Note that “λ” is an index of the air-fuel ratio expressed with reference to the theoretical air-fuel ratio, and is a so-called air excess ratio.
For example, JP2004-360593A discloses an art for executing, when a NOx stored amount in a NOx catalyst exceeds a given amount, a fuel injection control to enrich an air-fuel ratio of exhaust gas so as to reduce the NOx stored in the NOx catalyst.
One example of setting methods of an air-fuel ratio of exhaust gas so that NOx stored in a NOx catalyst becomes reducible (hereinafter, this air-fuel ratio is referred to as “target air-fuel ratio”) is performing a post injection after a main injection. In the main injection, fuel is injected into a cylinder so as to output a desirable engine torque, and in the post injection, fuel is injected at a timing so that the engine torque output is not influenced (typically, on expansion stroke). The fuel injected in the post injection typically needs to be combusted inside the cylinder in order to perform the reduction of the NOx catalyst, otherwise unburned fuel is discharged and degrades emission performance of HC (hydrocarbon), etc.
However, depending on the operating state of the engine, combusting the fuel injected in the post injection of the NOx reduction control inside the cylinder causes smoke (soot) or HC. For example, within a high engine load range, an in-cylinder temperature rises and the fuel injected in the post injection is ignited without sufficient time from the injection, i.e., combustion occurs before air and fuel are properly mixed, and thus smoke may be generated. On the other hand, within a low engine load range, even if the fuel injected in the post injection is to be combusted, since the in-cylinder temperature is low and combustion stability is low, the injected fuel may not properly be combusted, i.e., misfire may occur. In this case, HC corresponding to unburned fuel is generated. Further, within the low engine load range, since the temperature of the NOx catalyst is low, even if the air-fuel ratio is controlled to the target air-fuel ratio, reduction of the NOx catalyst may not be performed adequately.